Investigating Aerobic Respiration in Germinating Seeds

Aim

This experiment investigates whether germinating seeds carry out aerobic respiration, by testing for three signs of the process: the release of carbon dioxide, the release of heat, and the uptake of oxygen. Boiled (dead) seeds are used as a control to show that any changes observed are due to living tissue actively respiring.

Variables

• Manipulated variable: the condition of the seeds, living germinating seeds compared with boiled (dead) seeds used as a control.
• Responding variable: turbidity (cloudiness) of lime water over time, change in temperature recorded on a thermometer, and/or movement of a coloured liquid droplet in a simple respirometer.
• Controlled variables: mass or number of seeds used, size and insulation of the apparatus, room temperature, and the length of time the apparatus is left.

Materials and apparatus

• Germinating peas or mung beans, and an equal mass of boiled (dead) seeds as a control
• Lime water
• Two conical flasks or vacuum flasks with airtight stoppers and delivery tubes
• Cotton wool
• Two thermometers
• Potassium hydroxide (KOH) solution
• A capillary tube with a coloured liquid droplet (respirometer)
• Retort stand and clamp

Procedure

1. Divide the germinating seeds and the boiled seeds into two equal groups by mass, and place each group loosely wrapped in moist cotton wool inside a separate conical flask.
2. Connect each flask by a delivery tube to a test tube containing an equal volume of fresh lime water, and leave both set-ups for the same period, such as 30 to 60 minutes.
3. Compare how quickly the lime water in each test tube turns milky.
4. To test for heat release, place the germinating seeds in one insulated (vacuum) flask and the boiled seeds in an identical insulated flask, each with a thermometer inserted through a stopper.
5. Record the temperature in each flask at regular intervals over the same period and compare the readings.
6. To test for oxygen uptake, place germinating seeds in a flask together with a small container of potassium hydroxide solution, which absorbs any carbon dioxide produced.
7. Seal the flask and connect it to a capillary tube containing a coloured liquid droplet, forming a simple respirometer.
8. Observe and measure the distance moved by the coloured droplet over a fixed period of time.
9. Repeat the oxygen-uptake set-up using boiled seeds as a control, and compare the droplet movement.

Expected results

Living germinating seeds show clear signs of aerobic respiration in all three tests, while the boiled-seed control shows little or no change, because boiling kills the seeds and denatures the enzymes needed for respiration. The table below compares the two set-ups.

Conclusion

The results show that germinating seeds respire aerobically. Lime water turns milky faster next to living seeds because they release carbon dioxide, the temperature inside the flask of living seeds rises because respiration releases heat, and the coloured droplet moves towards the living seeds because they are taking in oxygen from the sealed air space.

The boiled-seed control shows little or no change in any of these three tests, which confirms that the changes observed with living seeds are caused by respiration in living tissue and not by some other process in the apparatus.

Safety

Common mistakes

Paper 3-style questions

Question 1 (hypothesis). A student connects a flask of living germinating peas to a test tube of lime water. State a hypothesis for what the lime water will do, and the result that would support it.

Model answer. Hypothesis: the lime water beside the living seeds turns milky because germinating seeds carry out aerobic respiration and release carbon dioxide. It is supported if the lime water next to the living seeds turns milky faster than the lime water beside the boiled-seed control.

Question 2 (variables and fair test). In the oxygen-uptake set-up, state the manipulated and responding variables and one variable that must be kept the same for a fair comparison.

Model answer. The manipulated variable is the condition of the seeds (living versus boiled control); the responding variable is the distance moved by the coloured droplet in a fixed time. A controlled variable is the mass of seeds used, so both set-ups start with the same amount of tissue.

Question 3 (tabulation and graph). The student measures how far the droplet moves every five minutes. Suggest how the results should be tabulated and what a graph of the results would show.

Model answer. The results are tabulated with time / min against distance moved / mm. A graph of distance moved (y-axis) against time (x-axis) gives a line that rises steadily, showing that oxygen is taken in at a roughly constant rate as the living seeds respire.

Question 4 (inference). In the boiled-seed control the droplet barely moves and the temperature hardly rises. What inference can the student make?

Model answer. The inference is that the changes seen with the living seeds are caused by respiration in living tissue, because boiling denatures the enzymes so the dead seeds cannot respire, take in oxygen or release heat. The results are therefore due to the seeds being alive, not to the apparatus.

Source:SRC-DSKP-EN

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